1. Field of the Invention
This invention relates to a filter-press type electrolytic cell wherein sodium hypochlorite is produced by diaphragmless electrolysis of saline water. Particularly, this invention relates to a saline water electrolytic cell which is effective to obtain saline water containing more than 10,000 ppm available chlorine with high current efficiency.
2. Description of Prior Art
Conventionally, chlorination has been used for the disinfection of water at waterworks and in sewage plants as well as in the oxidation of waste water. As a source of available chlorine, liquid chlorine or a sodium hypochlorite solution is used. However, these forms of available chlorine present some problems with respect to handling, economy and safety, so recently an electrochemical method wherein a sodium hypochlorite solution is generated by electrolysis of a dilute saline water like sea water has become popular.
To obtain a high concentration sodium hypochlorite solution by electrolyzing a dilute saline water, it is necessary to have a highly efficient method.
The inventors found that the use of the following anode in the saline water electrolytic cell increased current efficiency (approx. 80%) for production of sodium hypochlorite (10,000 ppm as available chlorine) and lowered cell voltage, thus reduced electric power consumption. Therefore, the advantages of the simple and compact structure coupled with a high current efficiency at a lowered cell voltage of the saline water electrolytic cell have been improved by the employment of an anode of a particular type as disclosed in Japanese Patent Publication No. 35473/1980. Such an anode comprises a substrate of titanium or an alloy thereof coated with a mixture consisting of a ternary mixture of platinum, palladium oxide, and ruthenium dioxide having a composition ranging from 3 to 42 wt % platinum, from 3 to 34 wt % palladium oxide, and from 42 to 94 wt % ruthenium dioxide on one hand, and from 20 to 40 wt % titanium dioxide based on the weight of said ternary mixture on the other hand.
In order to achieve further improved current efficiency for the production of sodium hypochlorite, a further study of the structure of the electrolytic cell is required.
In the conventional electrochemical methods for production of available chlorine, it is most economical to use a final available chlorine concentration of not more than 8,000 ppm, because as the concentrations of hypochlorite in electrolyte increases the current efficiency decreases as a result of increasing side reactions including an anodic oxidation and a cathodic reduction of hypochlorite ions. The current efficiency is further reduced either at too high or too low an electrolyte temperature. A temperature above 35.degree. C., especially higher than 40.degree. C., decreases the current efficiency about 10% or more, while a temperature below 15.degree. C., especially below 10.degree. C., lower the reaction rate of hydrolysis of chlorine to generate hypochlorite ions, reduces the current efficiency, and considerably shortens the life of an anode. Therefore, the temperature of the electrolyte must be kept within a range of from 15.degree. to 35.degree. C. However, the usual range of the dilute saline water supplied from a salt dissolving tank is from 2.degree. to 30.degree. C., and in summer the temperature of electrolyte may rise well over 50.degree. C. So far, no device has been offered which is properly equipped to control the electrolyte temperature within the above-mentioned range of from 15.degree. to 35.degree. C. In a batchwise process, a cooler is provided in the recirculating tank, and in other processes an external cooler is often provided outside the electrolytic cell; in any case, it is inevitable that the whole system including piping becomes large and complex.
The volume of gas generated by the electrolysis, mainly hydrogen, grows along the course of the electrolyte stream, impeding its flow and in an extreme case making electrolysis impossible. It may also reduce current efficiency and increase the electrolyte resistivity, thus increasing cell voltages and electric power consumption. Therefore, it is most important to provide an apparatus that enables smooth separation of gas bubbles with a reduced volume of gas in electrolyte.
It is desirable to have an apparatus that is compact and simple in structure, requiring only a small floor area for installation, and that is easy to maintain. The circulation tank process wherein saline water is continuously supplied and a solution containing about 8,000 ppm available chlorine is generated by recirculating a predominant portion of the electrolyte and discharging a lesser portion of the electrolyte is not advantageous, since the electrolytic cell is always operated at a high available chlorine concentration. For this reason, either a batchwise process or a continuous process is usually employed. On account of the larger size required for the former process, the continuous process is preferred. In the continuous process, it is required to provide a sufficiently long electrolyte path to assure a sufficient retention time in order to have a high final hypochlorite concentration. Various means have been devised to incorporate a desirably long electrolyte path in a compact electrolytic cell. An electrolytic cell disclosed in Japanese Patent Publication No. 28104/1977 comprises a large number of electrolytic compartments stacked one upon another, each consisting of a pair made up of a horizontally disposed screen cathode and anode, through which a saline water flow is conducted from the bottom upwards in series. Such a structure is complicated, particularly in the formation of an electrolyte path and in the provision of electrode supports. Another electrolytic cell disclosed in Japanese Patent Public Disclosure No. 50476/1980 comprises forming an electrolyte path by inserting an anode assembly and a cathode assembly through each of two opposing side walls of a box-type cell. However, it may sometimes be impractical to obtain proper interlacing of each member electrode plate of the anode and cathode assemblies, particularly when the number of such member plates is very large. The electric connection to be provided on both sides of the cell may give another difficulty. In any case, it is necessary to provide a cooling means to cope with a rise in electrolyte temperature. With this type of structure, it is impossible to incorporate a cooling means in the electrolytic cell. An external cooling means would mean a large complex system consisting of an array of electrolyte and cooling water pipings.
The filter-press type electrolytic cell of the present invention is given as an apparatus wherein the various problems described above have been solved by alternately placing an electrode plate possessing drilled holes to provide saline water passage and a gasket cut out at the center to provide an electrolytic compartment and by repeating this arrangement until a desired number of such electrolytic compartments is obtained. This type has the advantage of having a compact and simple structure. An example of a similar configuration was disclosed in Japanese Patent Public Disclosure No. 78675/1977. In this example, however, the electrolyte flows up and down alternately, so that the proportion of electrolytic gas in electrolyte (mainly hydrogen) increases as the concentration of hypochlorite increases, thus considerably disturbing the flow of electrolyte and adversely affecting electrolysis. Also another example disclosed in Japanese Patent Publication No. 38431/1980 does not take into account any increase in the gas content of electrolyte with a limited final hypochlorite concentration achievable and a complicated cell configuration due to a very narrow choice of possible electrode shapes.